The present invention relates to a device for moving and positioning an RF coil of an MRI (Magnetic Resonance Imaging or nuclear magnetic resonance-CT) device at an imaging position in a magnetostatic field.
With conventional X-ray CT (Computed Tomography) it is necessary, for determining the position of imaging an examinee, that a specified region of his body which is desired to be imaged be set at an optical marker fixed at a reference position. The optical marker is, for example, a cross-shaped light image projected onto the skin surface of the examinee.
Also in an MRI device of the type in which the RF coil is fixed at the center of a magnetostatic field generator, the above-mentioned specified region of the examinee lying on a movable pallet is set at an optical marker fixed at a reference position outside the magnetostatic field generator and then the pallet is moved a predetermined distance to bring the specified region to the center of the RF coil. In this instance, the RF coil must be made large to such an extent that the examinee carried on the pallet will not bump against the coil disposed in the magnetostatic field generator--this impairs the MR signal detecting sensitivity. Further, much labor and time are consumed for setting the optical marker at the specified region to be imaged while moving the pallet with the examinee thereon.
In an MRI device of the type in which an operator attaches the RF coil to the pallet in accordance with the specified region of the examinee's body on the pallet, the pallet is moved until the center of the RF coil attached to the pallet is brought to the vicinity of the position of an optical marker fixedly disposed, for example, at the inlet of the magnetostatic field generator, and thereafter the center of the RF coil is aligned accurately with the position of the optical marker, after which a drive start signal is applied to a pallet drive control unit to move the pallet a predetermined distance, thereby bringing the RF coil to the center of the magnetostatic field (i.e. the imaging position). In this case, since the RF coil is disposed in accordance with the specified region of the examinee's body to be imaged, the RF coil can be made relatively small; so that the MR signal detecting sensitivity can be increased. However, the performance of the MRI device is seriously degraded unless the center of the RF coil is accurately set at the center of the magnetostatic field. Accordingly, an appreciable amount of time is needed for setting the center of the RF coil at the position of the optical marker while carrying the examinee on the pallet as in the above-mentioned example, and in addition, much skill is required for such an operation.
In Japanese Patent Application Laid Open No. 1-284290 there is disclosed an MRI device which is free from the above-mentioned defects and permits easy positioning of a specified region of the examinee's body. In this device a slidable positioning piece is provided in a groove formed along one marginal edge of the pallet and a light sensor is disposed on an inner wall centrally of the magnetostatic field. The operator starts the pallet drive unit after sliding the positioning piece to set it at the specified region of the examinee's body on the pallet. The pallet drive unit stops the pallet from moving at the point of time when the light sensor optically detects the positioning piece. It is not described whether the RF coil is fixed at the center of the magnetostatic field or mounted on the pallet, but at any rate, the pallet carrying the examinee need not be moved for positioning the specified region of the examinee's body. In the case where this device is employed for positioning an RF coil, however, since the positioning piece is aligned with the specific region to be imaged through eye measurement, accurate positioning of the RF coil is difficult. Moreover, the method of stopping the pallet upon detection of the positioning piece is basically braking control, and in practice, the pallet does not always stop at a target position but its stopping position scatters according to the examinee's weight as well. In view of this, it is recommended in the afore-mentioned literature to selectively employ the optical marker or the positioning contact piece in dependence on whether the positioning accuracy required is high or not, but the individual methods which are selectively used still each have the afore-mentioned defect of its own.